Bond surface conditioning system for improved bondability

ABSTRACT

Thick film bond surfaces ( 8 ) on a support structure ( 10 ), such as a ceramic substrate or an IC package substrate, tend to deform during processing. A personality kit ( 16 ) having raised bosses ( 24 ) engages with and compresses the bond surfaces, resulting in a flatter, wider bond surface having improved reflectivity. The personality kit ( 16 ) is fit within a clamp ( 30 ) that can be used as a stand-alone unit or integrated into an existing machine, such as a wire bonder ( 46 ).

FIELD OF THE INVENTION

This invention relates generally to integrated circuit manufacturing andmore specifically to a wire-bonding step in the manufacture of anintegrated circuit package.

BACKGROUND OF THE INVENTION

As illustrated in FIG. 1, an integrated circuit chip 2 is electricallyconnected to the outside world by connecting a wire 4 (typically calledbond wire) from electrical contact points 6, commonly called bond pads,on the chip 2 to electrical contact points, commonly called bondsurfaces, 8 on a physical support structure 10. Conductive traces 12 onthe support structure then carry electrical signals to and from the chipto other components of the electrical circuit in which the chip isplaced. Typically, the traces and bond surfaces are formed of a thickfilm when the support structure is, e.g., a ceramic substrate, whichthick film is subject to heat in order to cure it. In applications wherethe support structure is a flexible film, thin films are more commonlyemployed for the conductive traces and, bond surfaces.

The integrated support structure 10 may be a lead frame in someapplications, which is subsequently encapsulated in a package, such asfor instance, by being placed in an injection mold and surrounded by aplastic encapsulation that electrically insulates and environmentallyprotects the chip, forming an IC package. Electrical contact to the chipcan be made via the lead frame, which extends beyond the package. Thefinished package is then mounted onto a circuit board to form part of adesired electrical circuit. In many applications, the lead framecomprises a thin, flexible film, such as polyemide, upon which is formeda thick film layer that is patterned to form the electrical traces 12.Ball grid array packages are a common example.

In other applications, the integrated circuit chip 2 is mounted directlyonto the substrate or circuit board, without the need for a leadframe.In such an application, the circuit board itself provides the supportstructure 10. Typically, the circuit board is a ceramic substrate uponwhich a thick film is formed and patterned to form the electrical traces12. One example of such an application is a multi-chip module in whichseveral integrated circuit chips are mounted on a common substrate andconnected together before being encapsulated. Another example is thecommonly called hybrid circuit in which several integrated circuit chipsare mounted directly onto a ceramic substrate (e.g., an aluminasubstrate) and electrically connected together by conductive traces 12on the substrate forming support structure 10. Typically, the entiresubstrate is then encapsulated in order to electrically isolate andenvironmentally protect the integrated circuit chips.

Regardless of the application, the bond pads 6 on the chip must beelectrically connected to bond surfaces 8 on the support structure. Thisis typically accomplished by connecting a thin wire, typically gold,between the bond pads and the bond surfaces, in a process referred to aswire bonding. In most applications, the bond wire is relatively thin, onthe order of 15-33 μ in diameter (although the teachings of the presentinvention is not limited by the size, type or composition of the bondwire). Typically, the wire is welded from the bond pad on the chip to abond surface on the support structure in a process referred to as wirebonding. Any type of suitable bond may be made at either the bond padsor the bond surfaces, including ball bonds, stitch bonds, and the like.The weld is typically performed using well know techniques such asthermosonic, ultrasonic, compression, and the like. A ball bond may beused, for example, at the bond pad and a stitch bond may be used, forexample, at the lead.

In many applications, the bond pads are also relatively small, on theorder of 2 mils to 10 mils square, although the teaching of the presentinvention applies to larger as well as smaller bond surfaces. Likewise,the bond surface is also generally on the order of several mils in size.Because the conductive traces 12, including the bond surfaces 8, arecommonly formed from a thick film, the bond surface of the bond surfaces(that is the surface to which the bond wire will be attached) is subjectto deformation—particularly during the bake process by which theconductive traces are cured after being formed on the support structure10. Materials such as gold, copper, nickel based alloys, aluminum,tungsten, copper-clad materials, and other well known alternatives aretypically employed for the conductive traces and bond surfaces.

FIG. 2 illustrates in cross section a bond surface 8 that has deformedduring the bake process. Note that, due to surface tension andshrinking, the top of the bond surface has taken on a pronounced radiusof curvature and that as a result, the width of the bond surface hasbeen reduced somewhat. Dotted lines 14 indicate the width of the leadsurface 8 prior to shrinkage. As shown, the actual surface available forplacement of a ball bond, stitch bond, or the like, is significantlyreduced after shrinkage—in some instances the width shrinks to 90% orless of the pre-shrinkage surface area. In addition, the radius ofcurvature of the lead surface also provides a less ideal surface ofbonding to than the ideal flat surface illustrated by dotted line 15.Both of these are deviations from the ideal bond surface, (i.e.shrinkage and radius of curvature) can result in the bond wire beingmis-aligned to the bond surface or an incomplete or unsatisfactory bondbetween the bond wire and bond surface. Device failure, performancedegradation, and reliability problems can result from such a situation.

In the past, device manufacturers have simply tried to minimize theeffects of bond surface deformation through selection of the leadmaterial and/or control over the heat processing of the device. Thisconstraints limit the flexibility and adaptability of the manufacturingprocess, however. Other past attempts to correct the problem haveinvolved an operator manually pressing down on individual bond surfaceswith a tool to attempt to flatten the leads. This process islabor-intensive, time consuming, and subject to wide process variations;it is not a practical solution for high volume, inexpensive, highreliability manufacturing.

It has been known in the past to employ a clamping device to hold leadframes in place during, e.g., the wire bonding process. Lead frames,upon which the bond surface may be found, are typically small, thincomponents, generally metallic, and are typically in the form of smallfingers extending from a central location near the chip outward to asurface to be connected to a circuit board. The lead frame fingers aresubject to damage and misplacement during the bonding process. Commonlyowned U.S. Pat. No. 6,322,659, issued Nov. 27, 2001 and entitled “Systemand Method for Dual Head Bonding,” which patent is incorporated hereinby reference, teaches such a method and apparatus for clamping leadframes in place during the bonding process.

Regardless of how lead frames are dealt with during the bonding process,what is needed is a method and apparatus that can quickly, reliably, andinexpensively condition a bond surface, particularly a thick filmsurface, to prepare the surface for reliable wire bonding. What is alsoneeded is a method and apparatus that can be readily integrated intoexisting manufacturing processes.

SUMMARY OF THE INVENTION

The above shortcomings of the prior art are overcome and novelcontributions to the art are provided by the preferred embodiments ofthe present invention. In one aspect, the invention provides for amethod of conditioning the bond surfaces of a support structure byaligning the support structure within a clamp such that the bondsurfaces on the support structure are aligned with raised boss surfaceson the clamp surface. The clamp is then brought into forcible contactwith the support structure such that the raised boss surfaces on theclamp contact and compress the bond surfaces, resulting in a flattenedbond surface. In the preferred embodiments, the clamp has a top elementcontaining the raised bosses that engage the bond surfaces on thesupport structure and a bottom element that engages the bottom of thesupport structure. The bottom element may or may not have raised bosses,depending upon the particular application. In some embodiments,particularly embodiments employed with a non-flexible support structure,the bottom feature of the clamp may not be necessary. Heat and/orvibration, such as ultrasonic vibration, can be employed in some aspectsof the invention, to make the bond surface more malleable. In someembodiments, the method occurs prior to the wire bonding operation. Inother embodiments, the method is integrated into the wire bondingprocess step.

In another aspect, the present invention provides for a device forconditioning the bond surfaces of a substrate. The device includes anon-flexible member upon which if formed raised bosses. The raisedbosses correspond to and align with bond surfaces on the substrate whenthe non-flexible member is brought into contact with a top surface of asubstrate. The device may also include a second member that is broughtinto contact with a bottom surface of the substrate. In someembodiments, the present invention may provide for a heater coupled tothe non-flexible member or the second member. In other embodiments, theinvention may provide for a vibrator coupled to the non-flexible memberor the second member.

In yet another aspect, the present invention provides for a bond surfaceconditioning machine including means for aligning a substrate to aconditioning clamp, means for forcibly engaging the substrate and theconditioning clamp such that raised features on the clamp forciblyengage with and flatter bond surfaces on the substrate, means forheating the bond surfaces, and means for attaching bond wires to thebond surfaces.

One advantage of the present invention is that yield and reliability inthe manufacturing process can be improved by providing for flat, uniformbond surfaces.

Another advantage of the present invention is that the resulting bondsurfaces tend to be smooth and reflective, thus allowing for improvedrecognition and alignment accuracy with, e.g., a machine vision system.

Yet another advantage of the present invention is that preferredembodiments can be realized as either a stand alone unit or can beincorporated into existing manufacturing process equipment, such asbeing incorporated into a conventional wire bonding machine with minimalretro-fitting.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates in perspective view an integrated circuit chip wirebonded to a support structure;

FIGS. 2a and 2 b illustrates in cross section view a bond surface on asupport structure;

FIG. 3a illustrates a preferred embodiment personality kit employingfeatures of the preferred embodiments of the invention;

FIG. 3b illustrates in cross sectional view a preferred embodimentpersonality kit and support structure;

FIG. 3c illustrates an alternative embodiment personality kit member;

FIG. 4 schematically illustrates a clamp mechanism employing features ofthe present invention;

FIG. 5 schematically illustrates a wire bonding machine in whichpreferred embodiments of the present invention may be employed;

FIGS. 6a and 6 b illustrate an alternative preferred embodimentpersonality kit used in conjunction with a ceramic substrate; and

FIGS. 7a and 7 b illustrate yet additional alternative preferredembodiment personality kits in which features of the present inventionmay be employed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3a illustrates a first preferred embodiment personality kit 16. Inthe first preferred embodiment, the personality kit comprises a topelement or member 18 and a bottom element or member 20. This embodimentwould be particularly useful for conditioning the bond surfaces of athin, flexible support structure 10 (FIG. 1), such as a polyemide orsimilar substrate commonly employed in surface mount IC packages such aball grid array (BGA) packages. FIG. 3b illustrates the personality kit16 in cross section as used with a support structure 10. As shown inFIG. 3b, the support structure 10, including an IC chip 2 mountedthereon, and thick film bond surfaces 8 formed thereon, is positionedbetween top member 18 and bottom member 20. Top member 18 preferably hasa cavity 22 formed therein, which cavity 22 is aligned with chip 2, soas not to disturb chip 2 when top member 18 is brought into contact withsupport structure 10. In other embodiments, the bond conditioning methoddiscussed herein can be applied to bond surfaces 8 prior to theplacement of IC chip on the support structure 10. In those embodiments,cavity 22 would not be required.

Top member 18 also has a series of raised surfaces, or bosses 24,located on its bottom surface. These bosses 24 are shown in phantom linein the perspective view of FIG. 3a and some of them are shown in thecross sectional view of FIG. 3b. As will be discussed in further detailbelow, these bosses 24 come into contact with and compress the bondsurfaces 8 in order to flatten and condition the bond surfaces surface.As best illustrated in FIG. 3b, each boss 24 is aligned with one or morebond surfaces 8 on the support structure 10. In some embodiments, bottommember 20 has corresponding bosses 25 as illustrated in FIG. 3c.

In operation, support structure 10 is placed between top member 18 andbottom member 20 with the bond surfaces 8 being aligned with the bosses24, as shown in FIG. 3b. Top member 18 is then lowered and bottom member20 is raised, so as to contact and compress the support structure 10.Particularly, because the bosses are raised features, the bosses willcontact the bond surfaces 8 and compress them slightly. Bottom member 20provides an inflexible base to ensure that support structure 10 (whichas described above may be a thin, flexible film) does not bend ordeform. It should be noted that both top member 18 and bottom member 20are preferably formed of rigid, non-flexible material to ensure that themembers will engage with the support structure 10 and bond surfaces 8 ina substantially flat uniform surface. This ensures that every bondsurface 8 is compressed uniformly and with equal compression.Preferably, the personality kit members are formed of industrial grademetal, such as a 440C stainless steel or comparable material. Rigidceramics could also be employed for the members. As discussed below,however, in some embodiments, top member 18 or bottom member 20 isheated prior to or during the compression step in order to heat the bondsurfaces 8. In such embodiments, the members should be formed of a heatconducting material. After the bond surfaces 8 have been conditioned byapplying compression for a pre-determined time, members 18 and 20 areopened, and support structure 10 can thereafter be removed for furtherprocessing, such as for wire bonding. One of ordinary skill in the artwill recognize that the amount of compressive force and the timeduration of compression will depend upon the material forming bondsurfaces 8, as well as the material forming support structure 10, asroutine experimentation and design choice. Additionally, the temperatureat which the compression operation takes place (i.e. whether the bondsurfaces 8 or the members 18 or 20 are heated prior to or during theoperation) can affect the time duration required to condition the bondsurfaces. Other factors, such as whether ultrasonic or thermosonicvibration is applied, can also impact the optimal compression force andtime duration, as will be readily appreciated by one of ordinary skillin the art.

FIG. 2b illustrates in cross sectional view a bond surface 8 after ithas been conditioned by being compressed in personality kit 16. Notethat, in contrast to FIG. 2a, the bond surface 8 surface is nowsubstantially flat. Note also that by flattening out the bond surface,the width of the bonding surface has been increased back to the designsize w. Additionally, the bond surfaces 8 on the support structure 10will have a substantially uniform height h after conditioning. This canbe advantageous in subsequent wire bonding operations. Anotheradvantageous feature of the conditioned bond surface is that the bondsurface will tend to be smooth and shiny. This provides for improvedreflectivity, which can be advantageous in the subsequent wire bondingoperations (particularly when such operations involve machine vision).

FIG. 4 illustrates a preferred embodiment clamp 30 in which personalitykit 16 may be employed. The clamp 30 includes to u-shaped arms 32 whichterminate with slots 34. Slots 34 receive the ends of upper member 18.Pins 35, located on the ends of arms 32 can be inserted through holes 36provided in upper member 18 (FIG. 3a) in order to secure the uppermember securely into the clamp. Arms 32 are connected to a movable base38, which base can move relative to a second movable base 40.Personality kit lower member 20 is attached to the second movable base40. In operation, base 40 will drive lower member 20 upwards while base38 drives arms 32 and hence upper member 18 downwards to engage with andcompress the support structure (not shown in FIG. 4). This motion isaccomplished by a drive and cam mechanism 42, the details of which arenot necessary for an understanding of the invention.

In the preferred embodiment, clamp 30 also includes a heater blockelement (not shown) is attached to base 40 and to which is attachedlower member 20. The heater block element serves to head lower member20, which then transfers the heat to the bond surfaces 8 during thecompression step. This heating of the bond surfaces allows for the bondsurface material (typically gold, copper, nickel, tungsten, aluminum, orsome other commonly employed conductive film) to be more malleable anddeformable without excessive compression. Heating the material alsolessens the chances of cracks forming in the bond surface material.

As one skilled in the art will recognize, clamp 30 can be driven in avariety of different manners to provide the movement of and compressionbetween upper and lower members 18, 20 of the personality clamp 16.Preferably, a servomotor is employed to drive bases 38 and 40, in orderto allow for precise movement and for precise control over the amount ofcompressive force applied to the bond surfaces. Other motor drives,including linear motors, stepper motors, and the like could be employed,as could cam driven pulleys, or pneumatic or hydraulic drive systems.

A wire bonding machine 46 employing preferred embodiments of the presentinvention is illustrated in FIG. 5. The machine illustrated in FIG. 5 isconfigured to operate in duplex mode, i.e. two devices can be operatedupon in parallel. To this end, machine 46 includes a first elevator 48in which a first group of devices to be operated upon can be loaded anda second elevator 50 in which a second group of devices to be operatedupon can be loaded. One skilled in the art will recognize that machine46 provides hardware and control circuitry and software to positiondevices (such as support structures 10) from the elevators 48 and 50 andposition the devices under bond capillaries 52 and 54, respectively.Although illustrated schematically for clarity, one skilled in the artwill recognize that bond capillaries 52, 54 entail numerous details forindexing and positioning the bond capillaries over the devices to bebonded, as well as for feeding through the bond wire, attaching the bondwire to the appropriate contacts on the IC chip and support structure,and the like. These details are omitted for clarity and are notnecessary to an understanding of the invention.

Because bonding machine 46 preferably operates on two devices inparallel, two clamps 30 are illustrated. Each claim holds a personalitykit 16, the top member 18 of which can be seen in FIG. 5. The bottommember 20 is obscured in the perspective view of FIG. 5. Also shown inFIG. 5 are mounting blocks 56 and 58. As described above, these mountingblocks may include heating elements that heat bottom member 20 ofpersonality kit 16, which heat is transferred to the bond surfaces 8during the compression operation. In alternative embodiments, blocks 56and 58 may provide the additional or alternative function of providingultrasonic vibration to the clamp, and hence to the bond surfaces to beconditioned. The blocks 56 and/or 58 can be driven by a piezo-eletricdevice, a linear or voice coil motor, or the like, configured to providevibrations at the desired frequency range, as will be apparent to oneskilled in the art. As discussed above, ultrasonic vibrations, inaddition to or perhaps in lieu of heat, will make the bond surfacematerial more malleable and deformable and hence more susceptible toconditioning.

The teachings of the present invention apply equally to more typicalsingle head bonding machines as well, in which case a single clamp andpersonality kit would be employed. Alternatively, a dual-head bondingmachine could be employed, in which one head performs bond surfaceconditioning and the other head performs the bonding operation. Oneskilled in the art will recognize that the teachings of the presentinvention also apply to a stand-alone machine that only performs thebond surface conditioning steps described above. Or the bond surfaceconditioning process described above could be incorporated into someother machine typically employed in the integrated circuit chip assemblyprocess flow.

While the preferred embodiments have been described with reference towire bonding operations, one skilled in the art will recognize theapplicability of the present invention to numerous processes forinterconnecting an integrated circuit chip to a substrate. Alternativeapproaches to the interconnect include flip chip process, tab connectionprocesses, bump bonds, conductive layer interconnects, and the like. Theteachings of the present invention will apply to any interconnect schemein which it is desired to condition an interconnect surface for improvedelectrical and physical connection, and preferably for improved visualrecognition by a machine, or the like. Therefore, the term bond surfaceshould be broadly construed to include any manner of interconnectsurface.

Another embodiment personality kit 16 is illustrated in FIG. 6a. Thisembodiment is preferably with a support structure 10 such as illustratedin FIG. 6b, which support structure is a ceramic substrate upon whichare placed one or more integrated circuit chips 2. Because the ceramicstructure is rigid and inflexible, it is not necessary to have a bottomelement 20 in this embodiment. The bottom surface of the top member 18is shown in FIG. 6a. Eight bosses 24 are illustrated. These bosses alignwith the various bond surfaces illustrated in FIG. 6b. Note that oneraised boss 24 can correspond to multiple different bond surfaces orbond sites. In other instances, a single boss, such as boss 24 a of FIG.6a will correspond to and contact only a single bond site.

Note also that no cavities 22 are shown for personality kit 16 member18. This is because this embodiment is intended to be used prior to thechips 2 being mounted on the substrate 10. This highlights the fact thatthe bond surface conditioning methods and apparatus discussed above canbe used either concurrently with wire bonding operations, or is aseparate, prior process step. One skilled in the art will recognize thatthe personality kit described above can be used as a stand-alone unit(preferably in combination with a clamp 30) or can be incorporated intoa special purpose machine or integrated into a machine having adifferent primary purpose, such as the wire bonder 46 illustrated inFIG. 5 and described above.

FIG. 7a illustrates an exemplary personality kit 16, with its bottommember 20 mounted on an exemplary block 56. As described above, block 56can include a heating element or a vibrating element to provide forthermal or ultrasonic treatment of the device clamped within thepersonality kit 16. Top member 18 has only a single cavity 22 forreceiving therein an integrated circuit chip, although it may haveadditional cavities 54 for securing the kit to a clamp or machine,alignment purposes, or the like. With reference to FIG. 7b, analternative personality kit 16 is illustrated in which top element 22has multiple cavities 22. In the illustrated embodiment, six suchcavities 22 are shown, although one skilled in the art will recognizethe number, size, and placement of the cavities are a matter of designchoice. The embodiment illustrated in FIG. 7b allows for conditioningthe bond surfaces of a substrate upon which has been placed severalintegrated circuit chips. Note that sufficient spacing between cavities22 must be maintained in order to allow for the formation of bosses 24between the cavities. Because the integrated circuit chips 2 willpresumably be spaced sufficiently far apart on substrate 10 to allow forbond surfaces to be formed between the chips, this limitation should notbe a problem. Whereas a single cavity 22 is shown for receiving eachchip, one or more cavities 22 could be made large enough to accommodateseveral chips therein.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

We claim:
 1. A method of conditioning interconnect surfaces of a supportstructure comprising: forming interconnect surfaces on the supportstructure; positioning said support structure board in alignment with apersonality kit such that the interconnect surfaces are substantiallyaligned with raised bosses on said personality kit; clamping saidpersonality kit onto said support structure such that the raised bossesforcibly engage with and deform each interconnect surface on saidsupport structure substantially simultaneously; and de-clamping saidpersonality kit from said support structure.
 2. The method of claim 1further comprising heating said personality kit.
 3. The method of claim1 further comprising vibrating said personality kit or said supportstructure during said clamping step.
 4. The method of claim 1 whereinsaid positioning step comprises aligning said support structure in atleast two axes.
 5. The method of claim 1 further comprising bonding abond wire to at least one of said interconnect surfaces after saidde-clamping step.
 6. The method of claim 1 wherein said clamping stepcomprises moving said personality kit, or said support structure, orboth, relative one another in a direction normal to the major surfaceuntil said personality kit and said support structure are forciblyengaged.